Natural killer (NK) cells and their receptors have been gaining prominence in the HIV field as more connections to HIV acquisition, progression, and viral evolution are being uncovered. One early finding in the field was that HIV-1-infected patients possessing the activating NK cell receptor KIR3DS1 and certain HLA class I alleles have a significantly slower progression to AIDS. Subsequent research has shown that KIR3DS1 triggers NK cell-mediated killing and cytokine production, and work from our group has demonstrated that KIR3DS1- expressing NK cells in co-culture with HIV-1-infected CD4+ T cells have a superior ability to suppress viral replication when certain HLA class I alleles are present. However, no one has been able to show an isolated interaction between KIR3DS1 and HLA class I. This has hampered our mechanistic understanding of how KIR3DS1 confers protection in HIV-1 pathogenesis and leaves open many possibilities as to what is triggering and interacting with KIR3DS1. Thus, the present goal of this study is to discover the KIR3DS1 ligand. To this end, we will implement a battery of unbiased, innovative, and complementary molecular biological and cellular immunological approaches - including use of soluble fusion constructs, reporter cells, and immunoprecipitation and mass spectrometry - to robustly identify the KIR3DS1 ligand. In addition, we aim to determine the cellular conditions that induce the expression of KIR3DS1 ligands in HIV-1 and other disease settings in which KIR3DS1 has been shown to be involved (e.g. other viral infections, cancer, autoimmunity, transplantation). This wil provide mechanistic insights as to how NK cell function is regulated through KIR3DS1:ligand interactions and, ultimately, elucidate how KIR3DS1 is able to slow HIV-1 disease progression, providing a new basis for NK cell-centered immunotherapeutic strategies that can help treat or control HIV/AIDS.